fix case-sensitive deps not resolving

`Scripts#lookup` starts by lowercasing the target name before doing its job but this causes some trouble if you need to require *a file name* with some mixed case letters in it.

The trouble is that this doesn't resolve to anything:

```js
require('repo/FileName')
```

Now, one way of working around this problem is to lowercase everything first, which in fact what builder is doing right now and most certainly doesn't bring the desired outcome from the developer's perspective. And it doesn't work either, because resolver.js doesn't know that.

A better option is to lowercase only user and repo fragments (this patch) 

Since we know user and repo are case insensitive, and must be case insensitive; we can safely translate the reference to lowercase and keep the rest untouched.